The present invention relates to the use of a reactor of plug-flow type to carry out a phosgenation process.
The use of a reactor of plug-flow or tubular type to carry out phosgenation processes is described for example, in the patent FR 1 578 808.
The phosgenation process is sometimes carried out in several stages. Thus, the application US 2006/0025556 describes a two-stage phosgenation process, the first stage preferably being carried out under high temperature and high pressure in a reactor of adiabatic plug-flow type. The application DE 102 60 094 describes a phosgenation process under high temperature and high pressure with successive reduction of the reaction medium in pressure in a reactor and reactive columns. The U.S. Pat. No. 7,112,694 reports a phosgenation process under high temperature and high pressure in a cascade of reactors of plug-flow type. The multiplication in the number of stages and of reactors for implementing the phosgenation renders complex the phosgenation process and the plant for the implementation. It is also more difficult to ensure the safety of personnel in the light of the complexity of the plants and of the multiplication in the number of components of the plants comprising phosgene, which is highly toxic by inhalation.
The phosgenation processes of the prior art also exhibit other disadvantages: the reaction volumes are generally high and undesirable products are formed during the reaction, which, on the one hand, prevent the fluids from flowing by formation of residues in the reactors and the pipes and, on the other hand, result in a fall in yield of the phosgenation reaction.
The works Turbulent Mixing and Chemical Reactions (Jerry Baldyga & John R. Bourne) and Géenie de la réaction chimique conception at fonctionnement des réacteurs [Chemical Reaction Engineering: Design and Operation of Reactors] (J. Viilermaux) report that a reactor of plug-flow type is particularly suitable for carrying out reactions of competitive type (namely, when at least two reactions are in competition with respect to one another)/consecutive type (namely, when at least two reactions follow one another). In point of fact, the phosgenation reaction of an amine corresponds exactly to this scenario since the isocyanate formed can in its turn react competitively with the starting amine to form a byproduct of urea type which cannot be recovered in value:R—NH2+COCl2→RNH—CO—Cl+HClR—NCO+2HCl: phosgenationR—NCO+R—NH2→R—NH—CO—NH—R: competitive formation of urea
The abovementioned works teach that the plug-flow reactor without back mixing is particularly suitable for carrying out the phosgenation reaction.
This is because, in a reactor of plug-flow type without back mixing, the molecules reacting with one another move in a single direction. Thus, the product formed, namely the isocyanate, does not return to the reaction region where the amine occurs. Consequently, the reaction for the formation of urea due to the reaction between isocyanate and the amine is penalized.